1. Field of the Invention
The invention relates to an optical device, in particular for a motor vehicle, such as a lighting or signaling device.
2. Description of the Related Art
A lighting or signaling device for a motor vehicle is known from patent application EP 1 881 263, which is equivalent to U.S. Patent Publication 2008/0019139 which is incorporated herein by reference and made a part hereof. The lighting device can emit a light beam “F” according to a longitudinal optical axis “A”.
This optical device comprises at least one sheet for guiding the light, at least one part of which is in the form of a portion of spherical cap.
The guide sheet is thus delimited in the direction of the thickness by two guide surfaces which are substantially parallel to one another along at least part of the sheet.
The guide sheet is delimited laterally by a front section for output of the rays of light, and by a rear section for reflection of the light. The ends of the reflection section are directly connected to the ends of the output section, such as to form the outer contour of the guide sheet.
The contour of the section for output of the light forms a flat arc of a circle.
The guide sheet comprises an aperture for intake of the light into the sheet.
A source of light is provide in the aperture, in the vicinity of, or in contact with the section for intake of the rays of light.
The source of light can emit rays of light according to a globally radial direction around a source axis “S” which is at right-angles to the guide sheet. More specifically, the source of light can emit a fan of rays of light radially, at least in the direction of the reflection section.
The guide sheet is made of a transparent material, the refraction index of which is greater than the refraction index of the medium in which the lighting device is designed to be immersed, for example, air. Thus, a ray of light which is introduced into the thickness of the sheet via its intake section meets the upper or lower guide surfaces with an angle of incidence relative to the normal line “N” which is greater than a limit refraction angle. The ray can thus be reflected totally by the guide surfaces.
The ray of light is thus guided in the thickness of the guide sheet by successive reflections between the two guide surfaces.
The incident rays of light which start towards the rear are designed to be reflected by the reflection section, then the rays of light thus reflected are directed towards the output section. The reflected rays of light thus emerge via the output section.
The source of light can be a so called side-emitter light-emitting diode or LED, which emits rays of light which are directed substantially perpendicularly to the source axis “S”.
The source of light can also consist of an incandescent lamp, for example, a halogen lamp with an axial filament, which is inserted in the contour delimited by the intake section. In the case when an area of the guide sheet in the vicinity of the intake section is made of glass, it is thus advantageously possible for the remainder of the sheet to be made of a plastic material which is over-molded onto this glass area. A design of this type makes it possible to eliminate the thermal problems which the use of an incandescent source could generate.
According to one embodiment of this patent application EP 1 881 263, the LED of a Lambert type is disposed in a aperture which opens only into one of the guide surfaces.
This LED is disposed such that its emitter surface is flush with the surface of the coupling area, which has been arranged such that the rays of light which are emitted by the LED are then redirected radially relative to the level of the coupling area.
The coupling area has locally an intake area in the form of a convex curved surface, on the surface of the side of which the LED is located, and, on the opposite surface and opposite this convex surface, it has an area with a form which approximates that of a cone.
The coupling area has symmetry of revolution.
This can make it possible to obtain performance levels which are comparable to the case of an LED of the side-emitter type.
However, the efficiency of a device of this type is reduced when the rear surface does not surround the LED sufficiently. This is the case, for example, when the dimensions of the rear surface are relatively small.
In this known device, taking into account the form of revolution of the area of coupling relative to the axis of the LED, the light is regularly distributed around the axis of the LED in the sheet.
The geometry of the sheet and of the rear surface makes it possible to distinguish several areas.
There is firstly an area in which the rays of light are directed towards the rear surface, and are returned cleanly forwards according to the optical axis.
There is then an area which corresponds to the rays which start directly towards the output surface of the sheet, and emerge from this sheet according to an angle which allows them to participate in the photometry of the beam.
Between these areas, the rays of light are either lost definitively, since they are reflected totally by the output surface, or they are refracted by the output surface towards excessively large angles which do not allow them to participate in the photometry of the beam.
Thus, the known device can be subject to relatively substantial losses of performance.
There is, therefore, a need to provide an optical device that facilitates reducing or eliminating substantial losses of performance.